(1) Field of the Invention
The present invention relates to a technique for a diversity process of an OFDM signal.
(2) Description of the Related Art
The OFDM (Orthogonal Frequency Division Multiplexing) is a technique of multi-carrier transmission of digital data and employed, for example, in ISDB-T (Integrated Services Digital Broadcasting-Terrestrial transmission) used in Japan and DVB-T (Digital Video Broadcasting-Terrestrial transmission) used in Europe.
FIG. 5 shows the frequency allocation of an OFDM signal.
The sub-carriers shown in the figure refer to carriers carrying data. For example, according to the ISDB-T/Mode 3 system conforming to the ARIB (Japanese Association of Radio Industries and Business) STD-B31 standard stipulating “Transmission System pertaining to Digital Terrestrial Television Broadcast”, the bandwidth of sub-carriers is 5.572 MHz, each spacing between the carriers is 0.992 kHz, and the total number of carriers is 5,617.
An OFDM transmitter adds carriers together called a guard band at both edge of the sub-carrier frequency band, so as to generate 8,192 carriers in total, which is a power of 2. The transmitter then generates an OFDM signal using Inverse Fast Fourier Transform (IFFT) of the 8,192 carriers at the sampling frequency Fs=8.127 MHz and transmits the OFDM signal.
Upon receipt from the transmitter, the receiver demodulates the received OFDM signal by orthogonal demodulation, Fast Fourier Transform (FFT), detection, error correction, and so on.
With the use of multiple sub-carriers to transmit data, OFDM is robust against multipath interference adversely affecting the frequency selectivity and thus exhibits excellent reception performance.
Yet, in a severe environment as in the case where the receiver is a mobile unit, the reception performance decreases. One way to improve the reception performance is a technique called diversity reception. To implement diversity reception, a plurality of antennas is used for reception. An OFDM signal received with the respective antennas is subjected to a diversity process to recover the quality of the received signal.
A conventional OFDM diversity receiver has separate receiving systems one for each antenna, and each receiving system independently demodulates the OFDM signal received with a corresponding antenna. Thus, a larger number of receiving systems inevitably gives a rise to a problem of a larger circuit size.
Generally, receivers envisioned to require diversity reception include mobile phones and car navigation systems, which are the products required to be as compact as possible. In view of this, there is a demand for reducing the circuit size.